Mixer circuit arrangement



Nov. 30, 1948. L.. LIBBY MIXER CIRCUIT ARRANGEMENTS Filed Dec. 27, 1944 WN N A 7' TUF/VE Y Patented Nov. 30, 1948 UNITED STATES PATENT OFFICE MIXER CIRCUIT ARRANGEMENT Lester L. Libby, East Orange, N. J., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application December 27, 1944, Serial No. 569,964

(Cl. Z50-20) 4 Ciaims. l

This invention relates to mixer circuit arrangements particularly of the type used for frequency conversion.

In certain conventional frequency-converting arrangements employing a mixer tube, such as those found in superheterodyne receivers, the received or incoming high frequency energy is combined with high frequency energy from a local oscillator to produce lower or intermediate frequency energy. In such receivers, the tuning of the input circuits of the receiver and the tank circuit of the oscillator are correspondingly varied so that the resultant intermediate frequency output of the mixer circuit is always a constant frequency.

Receivers of the type hereinbefore indicated may be designed to cover one or more relatively wide bands. In such receivers, it has been found that the lower frequencies of each band are discriminated against. This is in part due to the less favorable L/C ratio of the tuning circuits of both the receiver and local oscillator at the lower ends of each band than at the upper ends of each band. This fault is further accentuated at the higher bands when two or more bands'are employed and changeover from a lower band to the higher bands is accomplished by switching the inductance coils employed while utilizing the same condenser arrangement. In the higher bands, the L/C ratio becomes more unfavorable and the discrimination at the lower end of the higher bands relative to the upper end of each of said higher bands becomes even more marked.

An object of the present invention is the provision of a frequency conversion arrangement which will not discriminate against the lower frequencies of each band covered by said arrangement.

Said low frequency discrimination in such arrangements is to a great extent caused by the oscillator which injects into the mixer circuit lesser voltages at the lower frequencies of each band than at the upper frequencies thereof.

Another object of the present invention is the provision of a local oscillator and arrangement for injecting voltages into a mixer circuit which will not discriminate against the lower frequencies.

, Since, however, the input tuning circuits of the receiver will of themselves discriminate to some extent against the lower frequencies, it will be seen that despite the injection of equal voltages by the oscillator over all the frequencies of a given band, the output of the mixer circuit will vstill show some low frequency discrimination.

A further object of the present invention is the provision of a local oscillator and arrangement` for injecting voltages into a mixer circuit which will favor the lower frequencies. Thus, by favoring the lower frequencies in the oscillator and its injecting arrangement, the tendency to discriminate against the lower frequencies in the input circuits feeding into the mixer is balanced and a substantially flat output is derived from the mixer over all the frequencies of each band.

`Other objects of the present invention will become apparent and the foregoing will be best understood from the following description of an embodiment thereof, reference being had to the drawing, in which:

Fig. 1 is aschematic diagram of a circuit embodying my invention; and

Fig. 2 is a set of curves to which reference is made hereinafter, said curves not being intended to have any exact quantitative significance but merely serving as an aid in describing my invention.

Describing generally the mixer circuit arrangement of Fig. 1, potential from an alternating current source l, which may be an antenna or a radio frequency amplifier or the like, is applied through any suitable coupling means such as a coupling condenser 2 to the input circuit 3 of a mixer generally designated by the numeral 4. In the mixer 4, energy from the source I is mixed with energy from a local oscillator generally designated by the numeral 5. The entire arrangement is adapted to operate over a plurality of bands of frequencies and means are provided for correspondingly tuning both the input circuit 3 and theoscillator 5 with the oscillator tracking the input circuit. The resultant output of the mixer is an intermediate frequency.l f

Since there is a tendency on the part of the mixer, any previous radio frequency stages, and the oscillator to discriminate against the lower frequencies of each band, in accordance with my invention means are provided for counter-balancing this tendency. Said means comprises a tuned circuit 6 which couples the oscillator to vthe mixer and which circuit is tuned at the lower end of the band of frequencies of the oscillator output. As will be explained hereinafter, this causes the potentials applied to the mixer from the oscillator to be of greater amplitude at the lower end of each frequency band than at the upper end thereof.

Describing now in detail the arrangement of Fig. 1, the input circuit 3 of the mixer 4 includes a tuning condenser 1 and a pair of coils 8 and 9 3 across one of which the condenser 1 is connected by means of a switch arm I of a multiple switch arrangement Il, coil 8 being connected for the lower frequency band and coil 9 for the higher frequency band. One end I2 of the input circuit 3 is connected to the control element or grid I3 of an electron discharge device or tetrode I4 while the other end l5 of input circuit 3 is connected to ground and through a' cathode resistor IB to the cathode I1 of said tetrode. The screen grid I8 is connected through the usual screen resistor I9 to the positive end of a source of potential 29 whose negative end is grounded (not shown) in the conventional manner. The anode 2| of tetrode I4 is connected in series with the primary 22 of a transformer 23 and the usual anode resistor 24 to the source of potential 20.. The primary 22 is tuned by means ofa condenser 25 to the intermediate or beat frequency and the output of the mixer isf obtained from the secondary 26. of transformer 23. Conventional bypass condensers 21 and 28 are provided- The oscillator 5 illustrated inFig. l is a Hartley oscillator having a tank circuit 29. consisting of a tuning condenser 30, ganged to condenser 1, and apair of coils 3l and 32 connected in series with padder condensers 33 and 34. Switch arm 35 forming part of multiple switch II connects either of said coils and` its padder y condenser across condenser 30, coil 3| being connected for the lower frequency band and coil 32 for the higher frequency band. One. side 36-of condenser .3,0 is coupled through grid condenser 31 to the *control grid 38 of. a pentode 39, whilethe other side of condenser 30 is connected to ground. Bypass condenser 4I)A also couples the screen grid 4I of pentode 39 toA ground at radio frequency. Thesuppressor grid 42 is connected. in the conventional way inside the pentode to the cathode 431', which latter is in turn connected through switch arm 44 of multiple switch I L to an end ofthe taps 45 or 46 on coils 3l and 32 respectively. The usual grid resistor 4.1 is connected between control gridV 38 and, cathode 43.. The anode 4,8 `is connected in series; with a choke coil 49 having a tap 50, switch. arm 5l. of multiple switch II, anda conventional anode resistor 52 to the source of positive potential 20.. Likewise screen grid 4I is connected through a screen resistor 53 to said source 20.

From an understanding of the conventional Hartley oscillator, it will be apparent that the tan-k circuit 29 of oscillator 5 willi oscillate at a frequency controlled by the tuning of condenser 30 and depending on which of coils: 3lA and 32 is connected thereto,r and that alternating current potentials of such frequency will appear across choke coil 49. These potentials are injected or fedY into the mixer 4 byv being applied across` cathode resistor I6. thereof. For this-purpose, one end 54 of choke coil 49- is coupled through condenser 55 tov one side 5.6 of cathode resistor I5, whiletheother end 5-1 of choke coil 4S or the intermediate tap-15)- is connected through switch-arm.. 5I( andfcoupled; througlncon- -denser 58 to the other end 59 of cathode resistor E5. It will thus beseen that they oscillating potentials across choke coil 49 are applied across cathoderesistor I6 and thereby fed intothevmi-xer 4 and mixed withpotentials derived-fromsourceswitch arm. 5I, condenser 58;. resistor I5, con.-

4 denser 55 back to the other end 54 of choke coil 49. In addition there are various shunt capacitances in the circuit to the ground side thereof to which end 59 of cathode resistor I6 is connected. These capacitances include stray capacitances of the circuit elements to ground and interelectrode tube capacitances to ground such as, for example, the capacity of anode 48 of pentode 39, to cathode 43 thereof which latter is in turn connected through part of coils 3l or 32 to ground. The value of choke coil 49 and the positioning of tap 59 thereof is such that this tuned circuit 6 is tuned to the lower end of whichever band of frequencies the tank circuit of the oscillator covers or more specifically to the lower end of the band of frequencies of the oscillator output. As will be explained hereinafter, this causes the amplitude of the voltages applied across resistor I6 at the lower end of each band to be greater than at the upper end of each band.. Because of the resistor in tuned circuit 6, the circuit is of relatively low-` Q and the value of this Q can be determined by changing. the value of the resistor to provide the desired characteristic of variation of amplitude with change of frequency over a given band.

Referring now tc-Fig. 2 and particularly curves A, in which the horizontal axis represents frequency and the vertical axis represents the' required input voltage applied to theinput circuit of the mixer to produce a given output, and in which curve A1 represents the lower frequency band and curve A2 represents the higher frequency band, it will be seen that at the lower frequencies of each band, the requiredI input voltage is greater-than at the higher endI ofv each band. This is primarily due to the fact that the L/C ratio is less at the lower end of each band, that is, the C of the tunin-g condenser 1 is greater at the lower end of each band' than' at the upper while the' L is fixed. Moreover it will be seen in com-paring curve A1 representing the lower frequency band and curve A2 representing the higher frequency band that this discrepancy between the lower end of each bandi and the higher end of each band is accentuated in the case of the higher band. This same low frequency discrimination may occur in stages prior to the mixer circuit tool and thus increase the resultant total low frequency discrimination.

Not only is this discrimination against the lower frequencies apparent in the mixer circuit and prior stages but it is a-lso apparent in the conventional,r oscillator circuit as represented by curves B in which thel conventional oscillator output yvoltage is plotted against frequency. Curves B1 and. Ba roughly indicate how the output of the conventional oscillator is less at the lower end of eachband than at theupper end thereof.

To counter-balance this discrimination' against the lowerfrequencies in each band,` the output circuit of the oscillator is tun'edvfso that-ithas its maximum output. at the lower end .of each band. This output voltage is proportional to the impedance'of the tuned circuit and curves Czof Fig. 2' indicate the effect of tuningA said circuit to the lowerend-of one of said bands of= frequencies. By tuning tuned circuit 6 to the lower end of the upper band' of the oscillator ouput frequencies, the impedance of said circuit. will. vary asgindicated incurve C1 from; a maximum 60 adjacent the lower end of thefbandto ai. minimum 61` at/the upper end of the. band. ThisiimpedanceY curve can loe-adjusted by changing the resistance ofthe tunedl circuit E. toY producey a` steeper impedance curve such as, for example, curve C2, the steepness of this curve depending upon the Q of the circuit. When the impedance of the output circuit of the oscillator is as illustrated in curve C1, the resultant voltages injected into the mixer from the oscillator will be substantially as illustrated in curve B3 of curves B. Since the output of the mixer circuit depends on both the voltages received from the source and the injected oscillator voltages, the resultant output of the mixer will tend to be substantially level over the entire band as illustrated by curve A3 of curves A. This same correction will hold true for the lower band when the multiple switch Il is operated to connect in the various coils of the 10W frequency band and the added inductance between tap 5U and end 5'? of choke coil 9 so that the tuned circuit 6 is tuned to the lower end of the lower band.

While I have described by invention in connection with a specice oscillator and mixer, it will be apparent that other forms of oscillators and mixers may be substituted in place thereof as well as that the details of the described circuits may be varied without departing from the teachings of my invention. Accordingly, while I have described above the principles of my invention in connection with specic apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of my invention.

I claim:

1. A source of alternating current energy, an oscillator including a tank circuit, a mixer adapted to mix energy from said source and said oscillator, said mixer vhaving an input circuit coupled to said source, said input circuit and said tank circuit each being adapted to be tuned over a band of frequencies with the tank circuit tracking said input circuit, and a tuned circuit coupling the oscillator and mixer, said tuned circuit being fixedly tuned to a frequency at the lower end of the tank circuit band of frequencies.

2. The combination according to claim 1 wherein said mixer includes an electron discharge device having a cathode, and a grid coupled to one end of said input circuit, a cathode resister having one end coupled to the cathode and the other end coupled to the other end of said input circuit, and

6. wherein said tuned circuit includes a choke coil connected in the output circuit of said oscillator across which coil alternatingv current potentials are developed, and means for applying said alternating current potentials across said cathode resistor.

3. A source ofy alternating current energy, an oscillator including a tank circuit, a mixer adapted to mix energy from said source and said oscillator, said `mixer having an input circuit coupled to said source, said input circuit and said tank circuit each being adapted to be tuned over a plurality of bands of frequencies with the tank circuit tracking said input circuit, a tuned circuit coupling the oscillator and mixer, said input circuit and said tank circuit each including a tuning condenser and an inductance, and means for simultaneously switching in a different value of inductance in each of said input and tank circuits to select the band of frequencies over which each operates, and for fixedly tuning said tuned circuit to a frequency at the lower end of the selected band of frequencies of the tank circuit.

4. The combination according to claim 3 Wherein said mixer includes an electron discharge device having a cathode, and a grid coupled to one end of said input circuit, a cathode resistor having one end coupled to the cathode and the other end coupled to the other end of said input circuit, and wherein said tuned circuit includes a choke coil connected in the -output circuit of said oscillator across which coil alternating current potentials are developed, and means for applying said alternating current potentials across said cathode resistor, the aforesaid switching means being adapted to select the value of the inductance of said coil in accordance with the band of frequencies of said tank circuit.

LESTER L. LIBBY.

REFERENCES CTED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,001,695 Farnham May 14, 1935 2,235,576 Eisele et al Mar. 18, 1941 2,280,521 Foster Apr. 2l, 1942 

